Manually resettable electromagnetic indicators

ABSTRACT

A manually resettable electromagnetic flag indicator capable of binary indication comprising a sealed electromagnet sub-assembly and a display sub-assembly coupled together so that a pulse input to the electromagnet sub-assembly will cause the display subassembly to be operated to provide a first predetermined indication. The two sub-assemblies are coupled so that manual rotation of a portion of the display sub-assembly relative to the electromagnet sub-assembly will cause a second predetermined indication to be displayed.

United States [5 MANUALLY RESE'ITABLE 2,245,493 6/1941 Nothe ..340/373 ELECTROMAGNETIC INDICATORS 2,367,299 1/1945 McLam et a]. ..340/373 3 210 758 10/1965 Huston ..340/373 2 l [7 1 GwgeE Abmgmn Mass 3,573,811 4/1971 Knecht ..340/373 [73] Assignee: Miniature Electronic Components 3,185,979 5/1965 Jackman ..340/373 Corp, Holbrook, Mass. Primary Examiner-John W. Caldwell [22] Sept 1970 Assistant Examiner-William M. Wannisky [21] Appl. No.: 73,715 AttomeySchiller & Pandiscio i 7 ABSTRA [52] US. Cl ..340/373, 340/378 R, 335/125, [5 1 CT 33 5 219 335 220 A manually resettable electromagnetic flag indicator [51] Int. Cl. .Q. 6081) 5/22 Capable of binary indication comprising a Sealed elec- 58 Field of Search ..340/366, 381, 373,375,378; e Sub-assembly and a display sub-assembly 335/125, 219 220 coupled together so that a pulse input to the electromagnet sub-assembly will cause the display sub-as- 56] References Cited sembly to be operated to provide a first predetemiined indication. The two sub-assemblies are coupled so that UNITED STATES PATENTS manual rotation of a portion of the display sub-assembly relative to the electromagnet sub-assembly will 3,465,333 9/1969 P1hl ..340/373 d t d t b 3,487,403 12/1969 Pihl ..340/373 g s second me e e 1 e m o e 3,451,055 6/1969 Pihl ..340/373 3,573,812 4/1971 Pihl ..340/373 24 Claims, 9 Drawing Figures PATENTEUnuvza m2 SHEET 3 BF 4 POWER SOURCE GEORGE E P/HL //vv/v 70R.

5) g7 am/info ATTORNEYS PATENTED MW 28 I972 saw u 0F 4 ATTORNEYS.

MANUALLY RESE'ITABLE ELECTROMAGNETIC INDICATORS This invention relates to electromagnetic indicators and in particular indicators capable of binary indication.

The primary object of the invention is to improve upon electromagnetic indicators of the type known in the art as flag indicators.-

PRIOR ART ternate light'and dark sectors and is capable of rotation so as to provide different indications according to whether its light or dark sector arealigned with the transparent sectors of the stationary member. The movable indicator member moves between two limit positions and electromagnet means are provided for moving it to one limit position in which the indicator is considered to be set; the movable member is returned to the other limit position in which it is considered to be reset by the same or auxillary electromagnet means or movable mechanical means. Provision also is made for magnetically latching the movable indicator member in one or both of its limit positions. Indicator devices of the type described have utility in aircraft and other apparatus to indicate whether or not a fault or malfunction condition exists. Illustrative of the prior [art pertaining to indicators of the. type described are U.S. Pat. Nos. 3,406,388, 3,289,133, 3,234,436, 3,210,758, 2,367,299, and 2,245,493.

Heretofore the extent of use of indicators of the type described has been limited due to insufficient sealing against the environment, with reliability falling off rapidly when the units are exposed to temperatures, pressures and humidity conditions only slightly in excess 'of their expected capabilities. Prior attempts to provide improved sealing have not been satisfactory from the standpoint of reliability and other operating characteristics or because they have excessively complicated manufacture of increased costs.

THE INVENTION Accordingly, a more specific object of the invention is to provide an improvedelectromagnetic flag indicator wherein the components are effectively sealed so that the device can operate effectively over a long period of time under varying atmospheric conditions.

A furtherobject of the invention is to provide an improved flag indicator wherein the indicator members are sealed off from the electromagnetic drive, with the indicator members constituting part of one sub-assembly and the electromagnetic drive constituting pan of a separate sub-assembly. 7

Another object of the invention is to provide an improved flag indicator wherein one indication condition is achieved by energization of electromagnetic means and the other condition is achieved by manually operable means or by reverse energization of the same electromagnetic means.

Still another object is to provide a sealed flag indicator which is easy to assemble and which can be made at costs comparable to those of indicators of the same type.

The foregoing objects and other objects hereinafter rendered obvious are achieved by an indicator made up of two discrete sub-assemblies. One sub-assembly comprises a case with a pair of pennanent reset magnets and a pair of electromagnetsmounted therein. Each electromagnet has a core member of magnetic material that extends lengthwise of the case. Also mounted in the case is a hollow sleeve in which is rotatably disposed a part of the other sub-assembly. The latter sub-assembly comprises a ring in which is mounted a member which carries a pair of spaced magnetic stop assemblies and afirst shaft, a second shaft mounted within the first shaft so as to be capable of relative rotation; a flag magnet mounted on one end of the second shaft, and a first indicator member mounted on the flag magnet so s to rotate therewith. A second indicator member isattached to the ring. This ring assembly is rotatable coupled to the case with its first shaft disposed in the sleeve of the first describedassembly and ,springmeans are provided which act to bias the first shaft against rotation in a given direction. In the reset position the ends of the flag magnet engage the two stop assemblies which in turn engage the two core members. The electromagnets when energized cause the cores and the magnetic stop assemblies to repel the flag magnet and thereby cause the second shaft and the first indicator member to rotate to a set position where the flag magnet is again in engagement with the two stop assemblies and is magnetically latched thereto. Resetting the flag magnet is accomplished by rotating the ring relative to he case so as to bring the stop assemblies in engagement with the reset magnets, whereupon the flag magnet is repelled in a direction opposite to its original movement. The spring returns the ring to its original position when it is released by the operator. Magnetic shunt means are provided for preventing the electrical response of the device from being appreciably affected by the reset magnets. The reset magnets and the flag magnet are disposed with like poles on the same side of the device so as to provide stability against shock and vibration.

SPECIFIC EMBODIMENT OF THE INVENTION Other features and many of the attendant advantages of the present invention are set forth in or rendered obvious by the following detailed description which is to be considered together with the accompanying drawings wherein:

FIG. 1 is a longitudinal sectional view of a miniature electromagnetic indicator constituting a preferred embodiment of the invention;

FIG. 2 is a side elevation view, partly in section, of the same indicator taken from a vantage point removed from that of FIG. 1;

FIG. 3 is a plan view of a core return member used in the embodiment of FIG. 1;

FIG. 4 is a plan view of a printed circuit tabbing board used in the embodiment of FIG. 1;

FIG. 5 is a cross-sectional view taken along line 5-5 of FIG. 1;

I FIG. 6 is a front end view of the same device showing the nature of the display in the set condition;

FIG. 7 is a plan view of the movable indicator member, i.e. the flag, of the same device; 1

FIG. 8 schematically illustrates the operating circuit of the indicator; and 7 FIG. 9 is a schematic plan view showingthe relative operating positions of the flag magnet, the two limit stop assemblies, and the two reset magnets.

Tumingfirst to FIGS. 1 and 2, the illustrated embodiment of the invention comprises a case 2 which preferably is made of a non-magnetic metal. The case is provided with an exterior thread 4 for securing it to a supporting panel (such as theinstrument panel of an airplane cockpit) and also an exterior flange 6 adjacent its front end. The interior of the case is provided with a shoulder as shown at 8 and its front end is bevelled as shown at 10 to facilitate installation of a reset ring 12 which is herein-after described in greater detail.

The case 2 serves to contain two diametrically opposed electromagnets identified generally at 14 and 16. Each of the I electromagnets comprises a bobbin 18 made of a suitable insulating material, e.g., a plastic. Wound on each bobbin is a coil 20. Disposed within the hollow center portion of each bobbin is a soft iron core of circular crosssection, with the core for the electromagnet 14 identified by the numeral 22 and the other core identified by the, numeral 24. Each of the cores is formed with a flange 26 adjacent its bottom end which engages the bottom end of the bobbin in which it is disposed. The flanges 26 are disposed in appropriately spaced holes 30 formed in an annular plastic printed circuit tabbing board 32(see FIG. 4). The latter has two spaced circular copper lands 31, a third extended length copper land 33, two square holes 35 to accommodate the two reset magnets hereinafter described and two small holes 37 to facilitate connection of terminal leads 39 and 41 to lands 31. The printed circuit tabbing board engages the bottom ends of the two bobbins 18, while the other side of the tabbing board is engaged by an annular soft iron core return in the form of a disc 34 (see also FIG. 3). The core return disc 34 is provided with a pair of appropriately spaced holes 36 which snugly receive the bottom ends 38 of the two cores 22 and 24, plus two small holes 43 which are aligned with holes 37 of the tabbing board.

Also mounted within the case 2 is a member identified generally by the numeral 40 which functions as a sleeve bearing and also as a spring housing. The member 40 comprises a cylindrical hollow sleeve 42 whose bottom end is integral with an enlarged inverted cup 44. The member 40 is non-magnetic and preferably is made of a rigid plastic ,or black anodized aluminum. Its sleeve portion 42 extends through the center holes in tabbing board 32 and core return 34. The inverted cup 44 is provided with a radial slot 46 which functions to retain one end of a spiral spring 48 which is hereinafter described in greaterdetail. The bottom end of the cup is closed off by acover member 50 which is secured in place by any convenient means such as cement or by fasteners driven into the cup portion 44.

Also mounted in the case 2 are two elongate reset magnets 56 and 58 of square cross-section (see FIGS. 1, 2, 5 and 9). These magnets extend lengthwise of the case and are disposed in diametrically opposed relation to each other; additionally the two reset magnets 56 and 58 are each spaced 90 from the two cores 22 and 24. The bottom ends of the two reset magnets 56 and 58 extend through the openings 35 in the tabbing board 32 and engage the upper surface of core return disc 34. The upper ends of the two reset magnets and the upper ends of the two cores 22 and 24 are held in the abovedescribed spaced relation by means of a non-magnetic annular disc 60 which is seated on the shoulder 8 of the case. The center hole in disc 60 is sized to make a snug fit with the sleeve portion 42 of member 40. Disc 60 has two appropriately spaced round openings 61 and two square holes 63 to accommodate the two cores 22 and 24 and the two reset magnets 56 and 58 respectively. In this connection it is to be noted that at the upper ends of cores 22 and 24 half sections have been removed so as to provide oppositely facing flat surfaces 62 and 64. The two cores and the two reset magnets extend through disc 60 and their upper ends terminate substantially at the level of the bevelled end 10 of the case.

The upper end of the foregoing assembly is sealed off by application of suitable potting compound (not shown) to the junctions of disc 60 with case 2, the two cores 22 and 24, the two reset magnets 56 and 58, and the sleeve portion 42 of member 40. The bottom end of the same assembly is hermetically sealed by application of potting compound around the leads 39 and 41 so as to fill the space between core return 34, the cup 44 and the inner surface of the case 2. Additional potting compound may be applied over the cover 50 for the spring cup 44 as shown.

The ring 12 forms part of a separate sub-assembly. The upper or front end of ring 12 is formed with an inwardly extending flange 72 which defines an aperture that is closed off by a windowpane 74 formed of a transparent material, preferably glass, that constitutes an indicator member and is adapted to coact with another indicator member 75 (hereinafter described) to selectively display two separate conditions. As seen in FIG. 6, the windowpane 74 has three equally spaced opaque sections 76 formed by coating it with suitable dark colored, (e.g., black) paint, leaving three equally spaced transparent sector shaped areas 78. The three opaque sectors are joined in the center as shown at 79. The pane 74 is cemented directly to the flange 72 or is pressed against a sealing member 73 by a cup 80 which is made of a non-magnetic material and whose bottom wall 84 is essentially a flange formed integral with a cylindrical shaft extension 86. The shaft extension 86 is provided with a kerf 88 at its bottom end. The shaft 86 is hollow throughout a substantial portion of its length, its hollow portion having an inner diameter such as to snugly receive a shaft 92 hereinafter described in greater detail. The cup 80 is secured (by cementing or by a press fit) to the interior surface of the ring 12 so as to rotate therewith. The cup 80 functions as a chamber in which is disposed the second indicator member 75 which coacts with the pane 74 to provide a binary indication. As seen in FIG. 7 the indicator member 75 (also called the flag member) is a circular disc made of nonmagnetic material. The upper surface of indicator member 75 is divided into three equal white sectors 98 and three equal dark (e.g., black) sectors 100. The flag member 75 overlies the end of and is attached to a hub 100 formed on the end of shaft 92. The hub 100 is seated on a ring washer 102 which surrounds the shaft 92 and engages the bottom wall 84 of cup 80. The cup 100 is provided with a diametrically extending slot in which is secured an elongate bar magnet 104 which is hereinafter termed the flagmagnet. The flag member 75 overlies the flag magnet and is preferably secured to it and the hub 100 by a suitable cement. Flag member 75 has a rounded protuberance 107 that engages the pane 74 and acts as a thrust bearing to prevent upward movement of shaft 92. To the extent already described, it is believed to be obvious that shaft 92, magnet 104 and indicator member 75 constitute a discrete unit that is capable-of rotating relative to cup 80 and its shaft 86. Similarly, ring 12, pane 74, and cup 80 with its shaft 86 form a discrete unit that is capable of rotating relative to the case 2 and the member 40.

Mounted in the bottom wall 84 of the up are two assemblies 106 and 108 (hereinafter termed stop assemblies) which function as mechanical stops for the flag magnet and ring 12 and also as magnetic core extensions of cores 22 and 24 and reset magnets 56 and 58. These stop assemblies are identical except that they are reversed with respect to one another as described below. The two stop assemblies are made of a magnetic material such as mildsteel or other highly permeable magnetic alloy. Each stop assembly comprises a pin section 1 that extends through and is anchored in the bottom end 84 of cup 80, plus an upper transversely extending elongate stop section 112. The pins 110 extend below the cup 80 far enough to engage the cores 22 and 24 when the cup is in its at-rest position and the bar magnets 56 and 58 when the cup is rotated to reset the indicator as hereinafter described. Each pin 110 is cylindrical in cross-section but has a half section removed at its bottom end so as to provide a flat surface 114. The two stop assemblies are reversely oriented so that, as seen in FIGS. 1 and 2, the flat surface 114 of stop assembly 106 can squarely engage the flat surface 62 of core 22 while the flat surface 114 of stop assembly 108 can squarely engage the flatsurface 64 of core 24. The upper sections 1 12 of the two stop assemblies extend parallel to each other and extend far enough above the bottom wall of cup 80 to be engaged by the ends of the flag magnet 104 when the latter is rotated, but not so high as to interfere with the flag member 75. The end surfaces of flag magnet 104 are flat and at right angles to the magnets longitudinal axis. In contrast the ends of the upper portions 112 of the two stop assemblies are bevelled so as to form a pair of flat converging surfaces 118 at one end and a like pair of surfaces 120 at the other end (note that the two assemblies are oriented so that surfaces 118 of one and the surfaces 120 of the other are in corresponding positions in FIGS41 and 9). The two stop assemblies are spaced radially so that the side surfaces of the flag magnet can come into flat contact with the bevelledsurfaces 118 and 120. However, the bevelled end surfaces of the stop assembly are coated with a non-magnetic type 430 stainless steel and is elongate as shown in FIG. 9. Shunt member 128 has a center hole to accommodate theshaft 86 and it is secured to the cup 80. Shunt member 128 is disposed so that its longitudinal axis is aligned with the corresponding axes of the sections 112 of stop assemblies 106 and 108. Its length is such that, depending upon the position of ring 12, its ends will extend over and be spaced from the reset magnets 56 and 58 or the cores 22 and 24.

The ring assembly is secured in place by means of a washer 130 and a disc 132 which fit over the bottom end of shaft 86 within the spring cup 44. The washer 130 functions as a spacer and may be made of any suitable material. Preferably it is made of Teflon so as to reduce friction. The disc 132 acts to retain shaft 86 so that it cannot be pulled out of the cup 44. Ring 132 may be secured to shaft 86 in any convenient manner. Preferably it is a Waldes Truarc ring, which is well known topersons skilled in the art. This type of ring makes a friction grip with the shaft sufficiently strong so that it will not slip on the shaft. Completing the device is the helically wound spring 48. The inner end 134 of spring 48 is positioned in the kerf 88 of the shaft 86, while the outer end 136 of the spring extends into and is captivated by the slot 46 of cup 44.

The coils of the two electromagnets are connected in series with each other so that effectively they are oppositely wound on their respective cores. One end of one coil is soldered to the land 31 and another end of the coil is'soldered to the other land 31 of the tabbing board. The other ends of the two coils are connected to each other by soldering them to the ends of the large land 33 of the tabbing board.The two terminals 39 and 41 extend through the openings 37 and 43 in tabbing board 32 and core return 34 and are soldered to lands 31. In practice, the unit is mounted so that a current pulse can be applied between the terminals 39 and 41. As seen in FIG. 8 this may be accomplished by connecting the terminals to opposite side of a DC. power source 140 by way of. a switch 142 which may be a mechanical switch or a solid state switch. The two coils are energized by closing switch 142 and are deenergized' when the switch is reopened.

It is to be noted that the two electromagnets and the cores 22 and 24 may be considered to be a single electromagnet with the two cores 22 and 24 and the core return 34 forming a single U-shaped core. In this connection it is to be noted that only one coil is actually required to establish the desired polarities at the faces 62 and 64 of the two cores. However, two coils are preferred since less current is required to establish the required working magnetic force.

Operation of the foregoing device will now be described. Initially the ring 12 is positioned by the biasing action of spring 48 so that the flat surfaces 114 of the two stop'assemblies 106 and 108 engage the flat surfaces 62 and 64 at the upper ends of the cores of the two electromagnets l4 and 16. Assuming that the device has previously been reset, the flag magnet 104 will be in engagement with oppositely disposed Tefloncoated bevelled surfaces 1 18 of the two stop assemblies (the position shown in full lines in FIG. 9), and the shunt member 128 will extend over the two reset magnets. In this reset position the flag'75 will be disposed so that its black sectors will be aligned with the trans- 7 parent sectors of the windowpane 74. Hence, when the indicator is in its reset condition, an all black indication is provided. Assume also that the terminal leads 39 and v 41 of the indicator are coupled to power supply 140 through a switch and that the switch is momentarily closed so as to provide a current pulse which flows through the two series connected coils 20. Since the two coils are-oppositely wund, they will generate oppositely poled magnetic fields. Assuming that the flag magnet is in its first limit position where its north pole engages stop assembly 108 and its south pole engages stop assembly 106, the current applied to the two coils must be such that the upper end of the core 22 becomes a south pole and the corresponding end of the core 24 becomes a north pole. Stop assemblies 106 and 108 are now extensions of the north and south poles of cores 22 and 24 respectively. As a result, the flag magnet will be repelled away from the bevelled faces 1 18 of the two stop assemblies, and as seen in FIG. 9, will rotate clockwise to its second limit position (shown in broken lines) determined by its engagement with the bevelled faces 120 of the two stop assemblies. In this connection it is to be noted that as the flag magnet rotates clockwise, the repelling influence of the magnetic flux at the bevelled faces 118 will diminish with increasing distance and at some point the magnets continued movement will be due primarily to the attractive influence of the magnetic flux at the bevelled faces 120. Accordingly the current pulse need last just long enough to assure that the flag magnet will reach the point where the attractive influence of the flux at the bevelled faces 120 is strong enough to cause the flag magnet to reach its second limit position. It is to be noted that prior to application of the current pulse, the flag magnet is magnetically latched to bevelled surfaces 118 of the two stop assemblies. A similar latching action occurs when the flag magnet is in engagement with the bevelled surfaces 120 of the two stop assemblies. As a result of the change in position of flag magnet 104, flag member 75 will be oriented so that its white sectors will be lined up with the transparent sectors of the windowpane, thereby providing a black and white indication. Once the indicator has been set to provide this black and white indication, it will hold its set condition until it is reset by rotation of the ring 12. To reset the device, the ring 12 is rotated counter-clockwise as seen in FIG. 9. This movement of ring 12 will cause the two stop assemblies to rotate out of engagement with the cores of the two electromagnets and into engagement with the two reset magnets 56 and 58 (during this movement of ring 12, the flag magnet will be magnetically latched to and will rotate with the two stop assemblies). The reset magnets are disposed so that the top end of one is the north pole and the top end of the other is the south pole. With reference to FIG. 9, the upper end of the reset magnet 58 is the north pole and the upper end of the reset magnet 56 is the south pole. The ring is rotated counterclockwise against the biasing action of spring 48 until the pin 110 of stop assembly 106 engages the reset magnet 58 and the pin 1 of stop assembly 108 engages the reset magnet 56. When this occurs, the stop assembly 106 becomes a north pole extension of reset magnet 58, while the other stop assembly 108 becomes a south pole extension of the magnet 56. As a result the flag magnet is immediately repelled by the two stop assemblies and rotates counterclockwise back to its reset position in which it again engages the Teflon-coated bevelledv surfaces 1 18. When the ring 12 is released, it will immediately be rotated by spring 48 clockwise back to its original position in which the pins of the two stop assemblies are again in engagement with the cores of the two electromagnets. As the two stop assemblies rotate clockwise with the ring 22, the flag magnet 104 will remain magnetically latched thereto with the result that when the ring is back in its original reset position the black sectors of the flag member will again be aligned with the transparent sectors of the windowpane 74 so as to provide again an all black indication. Obviously, the windowpane 74 could be mounted in the ring 12 so that a black and white indication is provided when the device is in its reset condition and an all black indication is provided when the device has been placed in its set condition by application of a current pulse.

The function of the shunt member 128 is to limit the influence of the reset magnets 56 and 58 when the flag magnet is undergoing its excursion from its reset position to its set position, i.e., when the flag magnet is moving from its full line position to its dotted line position as seen in FIG. 9. Essentially the shunt provides a low reluctance path for the flux of the reset magnets so as not to disturb operation of the flag magnet by energization of the two electromagnets (since the shunt member is tied to cup 80, it moves out from over the reset magnets when ring 12 is rotated to effect resetting of the indicator).

It is to be noted that the device can be reset electrically instead of manually. This is accomplished by energizing the two electromagnets with a current pulse of opposite polarity, i.e., so that the upper ends of cores 22 and 24 have north and south magnetic polarities respectively. For this mode of operation the power source must be capable of selectively providing pulses of opposite polarity or switch 142 must be replaced by switch means capable of reversing the connections of power source 140 to the coils of the two electromagnets. By way of example, such switch means may comprise two single pole double throw switches that are mechanically ganged together for simultaneous operation, with one switch connected for selectively coupling the upper end of the coil of electromagnet 14 (as seen in FIG. 8) to the positive or negative terminal of the power source 140 and the other switch connected for reversibly coupling the lower end of the coil of electromagnet 18 (also as seen in FIG. 8) to the negative or positive terminal of the same power source, whereby current can be made to flow in one direction or the other through the two series connected coils 20.

Indicators constructed as above-described are characterized by fast response, low input power requirements, and relatively low sensitivity to shock, vibration, or the position in which the indicator is oriented when mounted in a supporting panel. It offers the advantages of an indication which is easy to discern under varying ambient light conditions and of being capable of construction in relatively small sizes, e.g., a case outside diameter of 0.62 inches and an overall length of about 0.85 inches. A further and very important advantage resides in the fact that the components mounted in the case 2 are effectively sealed off from 74 and cup 80 are also sealed off from the exterior environment. Because of such sealing and also because of the absence of any electrical components or critical mechanical movements in the space between cup 80 and plate 60, there is no critical need to provide an hermetic seal between the ring 12 and the case 4.An optional feature of the invention is to provide a viscous liquid in the space between the disc 60 and the bottom wall of the cup 80 so as to damp return rotational movement of the ring. A further advantage resides in the fact that the unit lends itself conveniently to assembling components of the device into at least three discrete sub-assemblies and also to testing the operation of the electromagnets before the device is completely assembled. It is also possible, by limiting the application of the potting compound 70 so that it does not conceal the cover 50, to uncouple the ring sub-assembly from the case 2 with a minimum of difficulty. Still other advantages will be obvious to persons skilled in the art.

What is claimed is:

l. A bistable electromagnetic indicator comprising a case, an electromagnet unit secured within said :case, said electromagnet unit comprising coil means and magnetic core means providing first and'second poles which are oppositely magnetically polarized when said coil means is energized, permanent magnet means secured within said case having third and fourth poles of opposite magnetic polarities, said first and second poles being disposed in alternately occuring relation to said third and fourth poles about the axis of said case,

first and second magnetic stop means spaced with respect to each other so that upon rotation as a unit on said axis they can move from engagement with said first and second poles respectively into engagement with said third and fourth poles respectively, means supporting said first and second stop means for rotation as a unit on said axis, a magnet mounted for rotation on said axis independently of rotation of said stop means, said magnet being polarized diametrically relative to said axis and being disposed so that said stop means are located in its path of rotational. movement and determine first and second limit positions for said magnet, cooperating indicating members for providing a first indication when said magnet is in said first limit position and a second indication when said magnet is in said second limit position, spring means for urging said first and second stop assemblies means into engagement with said first and second poles, terminal means for said coil means whereby said coil means may be energized to establish magnetic polarities at said first and second poles such as to cause said first and second stop means to magnetically repel said magnet from said first limit position to said second limit position, and means for manually rotating said first and second stop means as a unit out of engagement with said first and second poles and into engagement with said third and fourth poles, the polarities of said third and fourth poles being such as to cause said first and second stop means when in engagement therewith to magnetically repel said magnet from said second limit position back to said first limit position.

2. An indicator according to claim 1 wherein said means for manually rotating said stop means comprises a ring rotatably mounted on said case, and further wherein said means supporting said stop means is secured to and rotatable said ring.

3. An indicator according to claim 2 wherein said case, said electromagnet unit, said permanent magnet means and said temiinals means form a first discrete sub-assembly and said cooperating indicating members, said first and second stop means, said magnet and said ring constitute a second discrete sub-assembly that is separable from said first assembly.

4. An indicator according to claim 1 wherein said magnet is mounted on -the end of a first shaft and further including means for rotatably supporting said first shaft connected to said means for supporting said stop means.

5. An indicator according to claim 3 wherein said means for rotatably supporting said stop means for rotation on said axis comprises first and second rotatably connected members with said first member forming part of said first assembly and said second member forming part of said second assembly.

6. An indicator according to claim 5 wherein said first and second members are disposed in coaxial telescoping relation to each other.

7. An indicator according to claim 5 wherein said,

magnet is mounted on the end of a first shaft and further wherein said first shaft is rotatably supported by said first member.

8. An indicator according to claim 1 wherein said means for rotatably supporting said stop means comprises a first member having a shaft section and a peripheral flange section, and a second shaft member disposed in fixed relation to said electromagnetic unit, said shaft section and shaft member rotatably coupled together in axially aligned relationship, said stop assemblies being affixed to said flange section in radial spaced relation to said shaft section.

9. An indicator according to claim 8 wherein said magnet is mounted on the end of a shaft that is rotatably supported by said first member.

10. An indicator according to claim 9 wherein the shaft section of said first member has an axial bore and the shaft on which said magnet is mounted is rotatably disposed in said axial bore.

11. An indicator according to claim 8 wherein said spring is a helically coiled spring surrounding said shaft section within said case, one end of said spring being connected to said shaft section and the opposite end of said spring being anchored against movement with respect to said base.

12. An indicator according to claim 8 wherein said flange section is disposed between said magnet on the one hand and said electromagnet unit and said permanent magnet means on the other hand, and said stop means comprises first portions on one side of said flange and second portions on the opposite side of said flange, said first portions disposed to engage the poles of said core means and the poles of said permanent magnet means according to the rotational position of said stop means relative to the poles of said core means and said permanent magnet means andsaid second portions disposed to be engaged by said magnet in each of its two limit positions.

13. An indicator according to claim 1 wherein said electromagnet unit comprises two elongate cores and two coils with each coil wound on aseparate one of said cores, said coils being connected in series between said terminal means.

I 14. An indicator according to claim 13 wherein said cores are disposed in parallel spaced relation to each other and said axis.

15. An indicator according to claim 13 further including magnetic means connecting said two cores.

16. An indicator according to claim 13 wherein said permanent magnet means comprises two elongate permanent magnets disposed in parallel spaced relation to each other and said axis.

17. An indicator according to claim 16 wherein said elongate magnets and said cores are disposed about said axis in alternately occurring relation to each other.

18. An indicator according to claim 1 further including shunt means for providing a low reluctance path for said permanent magnet means when said magnet is moved by energization of said coil means.

19. An electromagnetic indicator comprising a rotatably mounted magnet, magnetic stop means for limiting movement of said magnet between first and second limit positions, an indicator member mounted for movement with said rotatably mounted magnet and adapted to provide a different indication in each of said limit positions, electromagnetic means operable when energized to rotate said magnet in a direction determined by the direction of current flow through said electromagnetic means, a permanent magnet reset assembly, said magnetic stop means being moveable relative to said permanent magnet reset assembly and said permanent magnet reset assembly being disposed so as to'be mechanically engageable by said magnetic stop means, and means for moving said magnetic stop means into and out of mechanical engagement with said permanent magnet reset assembly, said permanent magnet reset assembly being arranged so that when engaged by said magnetic stop means its magnetic field will cause said magnet and thereby said indicator member to rotate in a direction opposite to the movement thereof caused by energization of said electromagnetic means.

20. An indicator according to claim 1 further including damping means for damping rotational movement of said stop assemblies by said spring means.

21. An indicator according to claim 3 further including damping means for damping rotational movement of said stop means by said spring means.

22. An indicator according to claim 21 wherein said damping means is disposed between said first and second sub-assemblies.

23. An indicator according to claim 22 wherein said damping means is a viscous material.

24. An electromagnetic indicator comprising an indicator assembly that includes a magnet and an indicator member connected to move as a unit, support means rotatably supporting said indicator assembly, said support means including magnetic stop means for limiting rotational movement of said indicator assembly between predetermined limit positions, electromagnetic means operable when energized to magnetically urge said magnet to rotate in a direction determined by the direction of current flow through said electromagnetic means, a permanent magnet reset assembly, means connecting said support means and said reset assembly so that said support means is rotatable relative to said reset assembly, said reset assembly being disposed so that by relative rotation of said support means and said reset assembly said stop means are moveable into and out of proximity to said reset assembly, said reset assembly being adapted to exert a magnetic force of rotation on said magnet when said stop means are in proximity to said reset means and said indicator assembly is in one of said limit positions. 

1. A bistable electromagnetic indicator comprising a case, an electromagnet unit secured within said case, said electromagnet unit comprising coil means and magnetic core means providing first and second poles which are oppositely magnetically polarized when said coil means is energized, permanent magnet means secured within said case having third and fourth poles of opposite magnetic polarities, said first and second poles being disposed in alternately occuring relation to said third and fourth poles about the axis of said case, first and second magnetic stop means spaced with respect to each other so that upon rotation as a unit on said axis they can move from engagement with said first and second poles respectively into engagement with said third and fourth poles respectively, means supporting said first and second stop means for rotation as a unit on said axis, a magnet mounted for rotation on said axis independently of rotation of said stop means, said magnet being polarized diametrically relative to said axis and being disposed so that said stop means are located in its path of rotational movement and determine first and second limit positions for said magnet, cooperating indicating members for providing a first indication when said magnet is in said first limit position and a second indication when said magnet is in said second limit position, spring means for urging said first and second stop assemblies means into engagement with said first and second poles, terminal means for said coil means whereby said coil means may be energized to establish magnetic polarities at said first and second poles such as to cause said first and second stop means to magnetically repel said magnet from said first limit position to said second limit position, and means for manually rotating said first and second stop means as a unit out of engagement with said first and second poles and into engagement with said third and fourth poles, the polarities of said third and fourth poles being such as to cause said first and second stop means when in engagement therewith to magnetically repel said magnet from said second limit position back to said first limit position.
 2. An indicator according to claim 1 wherein said means for manually rotating said stop means comprises a ring rotatably mounted on said case, and further wherein said means supporting said stop means is secured to and rotatable with said ring.
 3. An indicator according to claim 2 wherein said case, said electromagnet unit, said permanent magnet means and said terminals means form a first discrete sub-assembly and said cooperating indicating members, said first and second stop means, said magnet and said ring constitute a second discrete sub-assembly that is separable from said first assembly.
 4. An indicator according to claim 1 wherein said magnet is mounted on the end of a first shaft and further including means for rotatably supporting said first shaft connected to said means for supporting said stop means.
 5. An indicator according to claim 3 wherein said means for rotatably supporting said stop means for rotation on said axis comprises first and second rotatably connected members with said first member forming part of said first assembly and said second member forming part of said second assembly.
 6. An indicator according to claim 5 wherein said first and second members are disposed in coaxial telescoping relation to each other.
 7. An indicator according to claim 5 wherein said magnet is mounted on the end of a first shaft and further wherein said first shaft is rotatably supported by said first member.
 8. An indicator according to claim 1 wherein said means for rotatably supporting said stop means comprises a first member having a shaft section and a peripheral flange section, and a second shaft member disposed in fixed relation to said electromagnetic unit, said shaft section and shaft member rotatably coupled together in axially aligned relationship, said stop assemblies being affixed to said flange section in radial spaced relation to said shaft section.
 9. An indicator according to claim 8 wherein said magnet is mounted on the end of a shaft that is rotatably supported by said first member.
 10. An indicator according to claim 9 wherein the shaft section of said first member has an axial bore and the shaft on which said magnet is mounted is rotatably disposed in said axial bore.
 11. An indicator according to claim 8 wherein said spring is a helically coiled spring surrounding said shaft section within said case, one end of said spring being connected to said shaft section and the opposite end of said spring being anchored against movement with respect to said case.
 12. An indicator according to claim 8 wherein said flange section is disposed between said magnet on the one hand and said electromagnet unit and said permanent magnet means on the other hand, and said stop means comprises first portions on one side of said flange and second portions on the opposite side of said flange, said first portions disposed to engage the poles of said core means and the poles of said permanent magnet means according to the rotational position of said stop means relative to the poles of said core means and said permanent magnet means and said second portions disposed to be engaged by said magnet in each of its two limit positions.
 13. An indicator according to claim 1 wherein said electromagnet unit comprises two elongate cores and two coils with each coil wound on a separate one of said cores, said coils being connected in series between said terminal means.
 14. An indicator according to claim 13 wherein said cores are disposed in parallel spaced relation to each other and said axis.
 15. An indicator according to claim 13 further including magnetic means connecting said two cores.
 16. An indicator according to claim 13 wherein said permanent magnet means comprises two elongate permanent magnets disposed in parallel spaced relation to each other and said axis.
 17. An indicator according to claim 16 wherein said elongate magnets and said cores are disposed about said axis in alternately occurring relation to each other.
 18. An indicator according to claim 1 further inclUding shunt means for providing a low reluctance path for said permanent magnet means when said magnet is moved by energization of said coil means.
 19. An electromagnetic indicator comprising a rotatably mounted magnet, magnetic stop means for limiting movement of said magnet between first and second limit positions, an indicator member mounted for movement with said rotatably mounted magnet and adapted to provide a different indication in each of said limit positions, electromagnetic means operable when energized to rotate said magnet in a direction determined by the direction of current flow through said electromagnetic means, a permanent magnet reset assembly, said magnetic stop means being moveable relative to said permanent magnet reset assembly and said permanent magnet reset assembly being disposed so as to be mechanically engageable by said magnetic stop means, and means for moving said magnetic stop means into and out of mechanical engagement with said permanent magnet reset assembly, said permanent magnet reset assembly being arranged so that when engaged by said magnetic stop means its magnetic field will cause said magnet and thereby said indicator member to rotate in a direction opposite to the movement thereof caused by energization of said electromagnetic means.
 20. An indicator according to claim 1 further including damping means for damping rotational movement of said stop assemblies by said spring means.
 21. An indicator according to claim 3 further including damping means for damping rotational movement of said stop means by said spring means.
 22. An indicator according to claim 21 wherein said damping means is disposed between said first and second sub-assemblies.
 23. An indicator according to claim 22 wherein said damping means is a viscous material.
 24. An electromagnetic indicator comprising an indicator assembly that includes a magnet and an indicator member connected to move as a unit, support means rotatably supporting said indicator assembly, said support means including magnetic stop means for limiting rotational movement of said indicator assembly between predetermined limit positions, electromagnetic means operable when energized to magnetically urge said magnet to rotate in a direction determined by the direction of current flow through said electromagnetic means, a permanent magnet reset assembly, means connecting said support means and said reset assembly so that said support means is rotatable relative to said reset assembly, said reset assembly being disposed so that by relative rotation of said support means and said reset assembly said stop means are moveable into and out of proximity to said reset assembly, said reset assembly being adapted to exert a magnetic force of rotation on said magnet when said stop means are in proximity to said reset means and said indicator assembly is in one of said limit positions. 